Depends upon your requirements. If you require just crack healing and the matrix is epoxy resin then an epoxy resin with higher functionality with a suitable amine will be the best Choice. If anticorrosive properties are also required along with self-healing then polyisocyanates are better option. If you can specify your system like matrix nature and your requirements then your answer can be specifically answered. Otherwise it's almost impossible to use a single healing agent for different matrix systems and requirements.